A finished gate structure (such as a finished gate or transistor gate) is the transistor terminal that modulates channel conductivity. Two principle approaches for forming semiconductor device gate structures are the gate-first and gate-last process approaches.
During fabrication of gate structures for, for instance, complementary metal-oxide-semiconductor (CMOS) technology, gate-first fabrication has traditionally been employed. In a gate-first fabrication approach, a conductor is provided over a gate dielectric, and then patterned (i.e., etched) to form one or more gate structures. After forming the gate structures, source and drain features of the semiconductor devices are provided.
More recently, the gate-last approach (or replacement metal gate (RMG) approach), has been employed. In the gate-last approach, a sacrificial (or dummy) gate material is provided and patterned (i.e., etched) to define one or more sacrificial gates. The one or more sacrificial gates are subsequently replaced with, for instance, a metal gate, after source and drain features of the devices have been formed. The sacrificial gate material holds the position for the subsequent metal gate to be formed. For instance, an amorphous silicon (a-Si) or polysilicon sacrificial gate may be patterned and used during initial processing until high-temperature annealing to activate the source and drain features has been completed. Subsequently, the a-Si or polysilicon may be removed and replaced with the final metal gate.
Although beneficial in certain aspects, conventional gate-last processing may be susceptible to silicon nodule defect formation on the sacrificial gate(s) during a subsequent epitaxial growth, for instance, on the fins to facilitate stressing the respective gate channels of the semiconductor structures being formed, referred to in the art as “FINFETs”. The FINFET takes its name from the multiple fins that form the respective gate channels of the field-effect transistors (FETs). This silicon nodule defect formation may be problematic, particularly as critical dimensions become smaller.